The present invention relates to an image forming apparatus such as a copying machine or a printer and a process cartridge detachably mountable thereto, and more particularly to a device for electrically charging an image bearing member using charging-promotion particles.
Referring first to FIG. 6, there is shown an example of a conventional image forming apparatus. The image forming apparatus of these examples is a cleanerless-type copying machine or a printer using an image-transfer-type electrophotographic process and a reverse development type process.
Designated by reference numeral 1 is an image bearing member in the form of a drum-type electrophotographic photosensitive member (photosensitive drum), which is rotated in the clockwise direction indicated by an arrow at a predetermined peripheral speed (process speed).
Designated by 2 is a charging roller (charging member) of a contact-type charging means, and is a solid or sponge roller of elastic electroconductive material. It is press-contacted to the photosensitive drum 1 with a predetermined pressure. Designated by a is a charging contact portion where the charging roller 2 is press-contacted to the photosensitive drum 1 (charging portion).
The outer surface of the charging roller 2 is coated with charging-promotion particles m, and therefore, in the charging contact portion a, there are charging-promotion particles m. The charging roller 2 is rotated in the clockwise direction indicated by the arrow, and in the charging contact portion a, the peripheral movement of the surface of the charging roller 2 is opposite from the moving direction of the surface of the photosensitive drum 1, and therefore, the surface of the charging roller 2 and the surface of the photosensitive drum 1 are in sliding contact with each other with a peripheral speed difference with the charging-promotion particles m therebetween. The charging roller 2 is supplied with a predetermined charging bias voltage, DC-600V, for example, from a voltage source S1.
By doing so, the outer peripheral surface of the rotating photosensitive drum 1 is electrically charged uniformly to a potential substantially equal to be applied charging bias voltage applied to the charging roller 2, through a direct injection charging process.
Subsequently, the surface of the photosensitive drum 1 having been uniformly charged is exposed to image light L at an exposed portion b by unshown image exposure means, by which the potential (light potential) at the light portion of the surface of the photosensitive drum 1 attenuates to provide a potential contrast between the light portion potential and the dark portion potential (dark potential). That is, the electrostatic latent image is formed on the surface of the photosensitive drum 1, corresponding to the image exposure pattern.
Designated by 3 is a reverse-developing device using a developer T containing magnetic one component insulative toner (negative charged toner). Developer T is externally added with a predetermined amount of charging-promotion particles m. A thin layer of the developer T containing the charging-promotion particles m is formed on the surface of the rotatable developing sleeve 3a and is carried into a developing zone c where the developing sleeve 3a is closely opposed to the surface of the photosensitive drum 1.
The developing sleeve 3a is supplied with a predetermined developing bias voltage from a voltage source S2.
By doing so, the electrostatic latent image on the surface of the rotatable photosensitive drum 1 is developed with the negative charged toner (reverse development) at the developing zone c, by deposition of the toner on the light portion of the surface of the photosensitive drum 1. The charging-promotion particles m contained in the developer are supplied to and deposited on the surface of the photosensitive drum 1.
Designated by 4 is a transfer roller as transfer means. It is contacted to the surface of the photosensitive drum 1 with a predetermined pressure, and is rotated such that it codirectionally moves with the surface of the photosensitive drum substantially at the same peripheral speed. Designated by d is a transfer nip where the transfer roller 4 is press-contacted to the photosensitive drum 1. Into the transfer nip d, a transfer material P (recording material) is supplied at a predetermined timing from an unshown sheet feeder and is fed by the transfer nip d. The transfer roller 4 is supplied with a predetermined transfer bias voltage from a voltage source S3.
By doing so, the toner image is continuously transferred (electrostatic transfer) from the surface of the photosensitive drum 1 onto the surface of the transfer material P which is being fed by the transfer nip d
The transfer material P having passed through the transfer nip d is separated from the surface of the rotating photosensitive drum 1 and is introduced into an unshown fixing device, where the transferred toner image is subjected to a fixing process The transfer material is discharged to the outside of the machine as a print or copy.
In this example, the image forming apparatus is a cleanerless type in which there is not provided a cleaner (cleaning device) exclusively for removing a small amount of residual toner remaining on the surface of the photosensitive drum 1. The untransferred toner, that is, the residual toner remaining on the surface of the photosensitive drum 1 after the image transfer operation, is carried again into the developing zone c by way of the charging contact portion a by the continuing rotation of the photosensitive drum 1, and is removed by the developing device 3 (simultaneous development and cleaning).
Since the charging-promotion particles m contained in the developer T in the developing device and deposited onto the surface of the photosensitive drum 1 during the developing operation, are electroconductive, and also since the charge polarity thereof is opposite from that of the developer (toner), they are substantially not transferred onto the transfer material P at the transfer portion d, and most of the charging-promotion particles m are kept deposited on the surface of the photosensitive drum 1. With the continuing rotation of the photosensitive drum 1, they are carried thereon to the charging contact portion a and are supplied thereto to provide always new injection sites. Therefore, even if the toner is accumulated on the charging roller 2, the deleterious result of improper charging can be avoided by supplying a large amount of the charging-promotion particles m.
A contact-type charging means using the charging-promotion particles m is disclosed in Japanese Laid-open Patent Application Hei 10-307454 and so on, and a structure with which the charging promotion particles are supplied to the contact portion between the charge member and the image bearing member from the developing means, is disclosed in Japanese Laid-open Patent Application Hei 10-307455 and so on. The contact-type charging means using the charging-promotion particles m is effective as a charging means for direct injection charging, uniformly to a predetermined voltage of a predetermined polarity, of the image bearing member (member to be charged) such as an electrophotographic photosensitive member, dielectric member for electrostatic recording or the like in electrophotographic image forming apparatuses or electrostatic recording image forming apparatuses.
The charging-promotion particles m are electroconductive fine particles for assisting the electric charging action, and it effects the direct injection charging by being present in the contact portion a between the charge member (charging roller 2) and the image bearing member (photosensitive drum 1). More particularly, the friction between the charging roller 2 and the photosensitive drum 1 at the charging contact portion a by the lubricating effect (friction reducing effect) of the charging-promotion particles m existing in the contact portion a so that the required rotation driving torque for the charging roller 2 is reduced. Therefore, the charging roller 2 and the photosensitive drum 1 can be contacted with a peripheral speed difference therebetween, and simultaneously, the charging roller 2 can be uniformly and closely contacted to the surface of the photosensitive drum 1 through the charging-promotion particles m. By this, the charging-promotion particles m rub the surface of the photosensitive drum 1 without a gap. Even if a simple member such as a charging roller, a charging sponge roller or the like is used, the ozoneless direct injection charging is made dominant with a low applied voltage.
Microscopically, the charging is uniform.
The direct injection charging fundamentally does not use the discharge phenomenon, but the electric charge is directly injected from the charging member into the image bearing member (member to be charged), by which the image bearing member is electrically charged, and therefore, the image bearing member can be electrically charged to the potential equivalent to the applied potential even if the voltage applied to the charging member is lower than the discharge starting voltage. Since no ion is produced due to the discharge phenomenon, and therefore, no damage due to the discharge product can be avoided.
In an image forming apparatus of an image transfer type, the residual developer (toner) remaining on the surface of the image bearing member after the image transfer, is removed from the surface of the image bearing member by a cleaner and is collected as residual toner, which however is not its idea from the standpoint of protection of the environment. Therefore, the above-described cleanerless image forming apparatus has been proposed in which the residual toner remaining on the photosensitive drum 1 after the image transfer operation is removed from the photosensitive drum 1 by the developing device 3 (simultaneous development and cleaning), and the toner is collected into the developing device 3 and is reused.
In the simultaneous development and cleaning, the toner remaining on the image bearing member after the image transfer operation, is removed in the subsequent developing operation. That is, the image bearing member is continuously charged and exposed to the light to form an electrostatic latent image, and during the developing operation for the latent image, the residual toner is removed by the application of the fog-removing bias voltage (a fog removing potential difference Vback which is a potential difference between the DC voltage applied to the developing device and the surface potential of the image bearing member).
With this method, the residual toner is collected into the developing device and is reused in the subsequent image forming process, so that residual toner is eliminated, by which the maintenance operation is made easier. Additionally, since the cleaner is not used, the space required thereby can be saved, so that the image forming apparatus can be significantly downsized.
In the toner recycling system, the residual untransferred toner is not removed by a cleaner provided exclusively therefor, but is reused by collecting in the subsequent developing operation. Therefore, in the case that contact charging is employed as charging means for the image bearing member, it is a problem how to electrically charge the image bearing member while the toner, which is electrically insulating, exists in the charging contact portion where the contact charging member contacts the image bearing member. In the case of roller charging or the furbrush charging, the untransferred toner on the image bearing member is scattered into a non-pattern, and a high-voltage is applied to effect charging using electric discharge, in many cases. The magnetic brush charging is advantageous in that the magnetic brush portion of the electroconductive magnetic particles, which are the contact charging member, are contacted softly to the image bearing member, but the structure of the charging device is complicated, and defects resulting from the loss of the electroconductive magnetic particle constituting the magnetic brush, is significant.
In the case of the contact-type charging means using the charging-promotion particles m, the charging-promotion particles m are present in the charging contact portion a, so that even if the untransferred toner is mixed into the charging contact portion a or even if the charge member 2 is contaminated with the toner, the close contactness and the contact resistance between the charge member 2 and the image bearing member 1 can be maintained at the charging contact portion a. In addition, even if the toner is accumulated in the charging contact portion a or on the charge member 2, improper charging can be prevented by supplying a large amount of the charging-promotion particles m.
In this type of device, the untransferred toner T is subjected to charge injection similarly to the image bearing member 1 while the untransferred toner T passes between the image bearing member 1 and the charging-promotion particles m in the charging contact portion a during the charging process, by which the proper charging of the toner is maintained, and therefore, when the toner passes through the developing zone region, the toner is collected back into the developing device 3 with certainty. Accordingly, the electrophotographic process can be carried out without a cleaner.
In the above-described image forming apparatus in which the charging means for uniformly charging the image bearing member 1 to a predetermined polarity or potential, and the charging-promotion particles m are supplied from the developing means 3 to the charging contact portion a where the charge member 2 and then the image bearing member 1 contact each other, the width (measured in the longitudinal direction) in which the charging-promotion particles m are supplied is the same as the width in which the development is carried out, because the charging-promotion particles m to be supplied are mixed in the developer T in the developing container of the developing device 3 and because the charging-promotion particles m are supplied together with the developer when the latent image formed on the image bearing member 1 is being developed. In other words, even if the weakest of the charging roller 2 which is the charging member is made longer than the developing with, the injection charging does not occur at a longitudinal position (end portion) to which the charging-promotion particles m are not supplied, and therefore, the charging width is equal to the developing width.
When the charging width is equal to the developing width, the following problems arise. FIG. 7 shows an example of potentials of the photosensitive drum (image bearing member) 1 and the developing sleeve (developer carrying member) 3a adjacent an end of the development width, in which Vd is a dark potential (approx xe2x88x92600V), and V1 is a light potential (approx xe2x88x92150V). Designated by Vdc is a developing potential (approx xe2x88x92300V).
Point (2) corresponds to the end of the charging region, and the potential there is Vd, and the toner is not deposited on the point (2) because of the potential difference (back contrast) between points (1) and (2). However, at point (3), the potential is substantially 0 V, so that it is as if the charge has been attenuated by the exposure to light, and therefore, the toner is deposited to the point (3) due to the potential difference (contrast) between points (1) and (3) (so-called xe2x80x9clateral jumpxe2x80x9d of the toner) with the result of contamination of the background on the transfer material.
In order to avoid the lateral jump of the toner, it would be considered that charging-promotion particles m are applied to a range wider than the developing width before the start of the use of the apparatus. However, if the maximum width of the transfer material is larger than the developing width, the charging-promotion particles outside of the development width in the longitudinal direction are transferred onto the transfer material in long-term use, so that the amount of the charging-promotion particles gradually decreases with the results of improper charging and lateral jump of the toner.
Accordingly, it is a principal object of the present invention to provide an image forming apparatus and a process cartridge detachably mountable thereto, in which the reduction of the electroconductive particles is suppressed, and condemnation of the transfer material attributable to the lateral jump of the toner is prevented.
According to an aspect of the present invention, there is provided an image forming apparatus comprising a movable image bearing member; charging means including a charge member for electrically charging the image bearing member with electroconductive particles between the charge member and the image bearing member; and developing means for developing a latent image with toner mixed with electroconductive particles, wherein the latent image is formed on the image bearing member having been charged by the charging means, and a toner image provided by development of the latent image by the developing means is transferred onto a recording material, wherein the electroconductive particles mixed in the toner is carried by the image bearing member and is fed to between the charge member and the image bearing member, and wherein the width A, measured in a direction perpendicular to a moving direction of the image bearing member, in which the electroconductive particle are present between the charge member and the image bearing member, the width B in which the developing means is capable of effecting development, and the recording width C of a maximum size recording material, satisfy A greater than Bxe2x89xa7C.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.